Isotope separating apparatus



Aug. 2, 1955 w c, TUNNELL 2,714,665

ISOTOPE SEPARATING APPARATUS Filed June 23, 1945 2 Sheets-Sheet l INVENTOR. WZZz'am C Tare/2622 Aug. 2, 1955 w. c. TUNNELL ISOTOPE SEPARATING APPARATUS 2 Sheets-Sheet 2 Filed June 25, 1945 INVENTOR. 67227272622 BY United States Patent Ofitice 2,714,665 i atented Aug. 2, 1955 2,714,665 KSOTOPE SEPARATING APPARATUS William C. Tunnel], Oak Ridge, Tenn., assignor, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Application June 23, 1945, Serial No. 601,115 16 Claims. (Cl. 250-413) This invention is concerned with improvements in isotope separating apparatus and particularly involves improved arrangements for maintaining the ion source in such apparatus in operative condition.

In known types of apparatus used for separating the for example in the type of apthe application of Ernest 0. Lawrence, Ser. No. 557,784, filed October 9, 1944, the apparatus has embodied therein an ion source comprising a chamber for volatilizing a material therein, the isotopes of which it is desired to separate, the chamber having an elongated opening or slit from which, or through which, the volatilized material emerges or is emitted during ionization thereof. During operation the elongated opening or slit may become fouled up or otherwise contaminated with condensate of the material or other contaminations, and since the geometry of the slit in relation to other parts is critical, it is imperative that the slit or opening be kept clean, that is, free of contaminating material. An object of my invention is to provide improved means for cleaning this slit in isotope separating apparatus of the type referred to for the purpose of maintaining it in a proper and eflicient operating condition. My invention is an improvement in the particular mechanical construction of the earlier application of Roswell C. Van Sickle, Serial No. 676,221, filed June 12, 1946.

Another object of my invention is to provide a slit cleaning mechanism for ion sources of the type referred to above embodying a straight line motion producing mechanism for causing the slit cleaning element to move in a straight line in the slit or elongated opening without binding or disalignment and without the presence of bulky or unwieldy guide members or other mechanisms in the immediate vicinity of the elongated opening of the ion source.

Another object of my invention is to provide a slit cleaner mechanism for a plurality of ion sources comprising a single operating mechanism arranged to produce a straight line motion of a plurality of slit cleaning elements and having means to prevent binding of the elements which might result particularly from one getting ahead of another.

Another object is to provide a slit cleaning mechanism as in the previous object wherein the straight line motion involves a lever having both back and forth up and down motion in a guide slot and means providing guiding surfaces spaced over a considerable area to prevent twisting or wobbling of the lever in the guide slot.

Further objects of my invention and numerous of its advantages will become apparent from the following detailed description and annexed drawing wherein Fig. 1 is a diagrammatic cross-sectional view of an isotope separating apparatus embodying an ion source with the slit cleaning mechanism of my invention associated therewith.

Fig. 2 is a diagrammatic sectional view taken along the line 22 of Fig. 1 showing the relationship of the ill) magnetic field producing means to the chamber within which the isotope separation action takes place.

Fig. 3 is a diagrammatic perspective view of the slit cleaning mechanism with the parts rotated to the right substantially through 90 from the position in which they are shown in Fig. 1, and with some of the parts partly broken away.

The apparatus of Fig. l is of the type disclosed in greater detail in the prior application of E. 0. Lawrence referred to above. The apparatus of Fig. 1 includes a tank 10, the interior of which is evacuated to a relatively high degree of vacuum as will presently be described. The tank 10 is shown in cross section, and one side is closed by a relatively heavy face plate 11 which is attached to the tank by means of screw clamps 12 and face plate 11 is clamped in position it engages the tank in sealing relationship so as to insure against loss of vacuum at the joint. All of the mechanism of the tank 10 is attached to and carried by the face plate 11 as will presently be explained.

The tank 10 has windows 14, 15, 16 and 17 so as to make it possible to watch operations within the tank and there is a large discharge outlet 20 through which the air and moisture in the tank are evacuated. The discharge outlet 20 is connected to a diffusion pump or pumps 21 and pipe 22 and the diffusion pump is connected to a mechanical pump or pumps 23 by a pipe 24. The mechanical pump 23 may be a Kinney pump, for 6X- ample, which is a rotary type of pump. In practice a liquid nitrogen trap may be used in the tank 10 for freezing out moisture, or such a trap may be associated with the pipe 22 connected to the dilfusion pump. Also, a trap using solidified carbon dioxide may be used in association with the pipe 24- for freezing out moisture from the air before it reaches the pump 23.

The tank 10 is disposed between laminated iron cores or pole pieces 27 as shown in cross section in Fig. 2 and surrounding these pole pieces are electrical windings 28, the windings being disposed within tanks or housings 2?, one of the housings 29 being shown in cross section in The windings 28 are energized with electrical current so that a relatively intense magnetic field is produced which is in a direction transverse to the tank 10, that is, in a horizontal direction looking at Fig. 2 and in a direction perpendicular to the paper looking at Fig. 1. A cooling medium such as cooled oil or the like is circulated through the housings 29 for the purpose of cooling the coils therein.

Within the tank 10 of an arcuate conduit which forms a passageway for a beam or beams of ions the liner and received in upper end of the liner.

a receiver or collector at the The liner 35 and the apparatus associated therewith are carried by the face plate 11 by means of a supporting structure designated by the numerals 36 and 37 and the brace member 33. The liner itself is attached to and spaced from the supporting structure by insulators 32, 33 and 34 since, as will presently be disclosed, the liner is maintained at a different potential than the supporting structure. Within the lower right corner of tank 10 there are a pair of castings 4t) and 40' which forms containers within which the material to be ionized is vaporized. The castings 40' and 46' may be supported from the supporting structure 36. The castings 4t) and 40', and the apparatus associated with each, are identical so that the description of one applies to the other as well.

The casting 40 comprises a lower portion 31 and a smaller upper portion 42, the upper portion being connected by a converging throat as shown. Within the portion 31 is a container or bottle 43 which is removable and in which the charge material itself is placed, that is, the charge of material to be ionized. Numeral 52 designates an electrical heating element or heating elements associated with casting which supply heat for volatilizing the charge material within the bottle 43. In the side wall of the smaller portion 42 of casting 40 there is a well 53 and the casting 40 has a similar well 53'. Electrical controls are provided for electrically controlling the supply of power for energizing the heating elements 52 and 52. The electrical controls for heating element 52 as shown are responsive to a thermocouple which is placed in the well 53' and the controls may preferably include the control known as the Micromax manufactured by the Leeds & Northrup Company; this control is designated by the numeral 54 and the thermocouple in well 53' is connected to it. The instrument 54' is connected to a control instrument 55' which is a type of instrument which operates to control the flow of electric power to the heating elements 52, and it may preferably be the control instrument known as the Reactrol manufactured by the General Electric Company. The control instrument 55' is connected to the heating elements 52' as shown.

Referring to the casting 40, at the left hand of portion 42 there is a longitudinal slit 5%, that is, an elongated opening through which the vaporized material passes. The vaporized material passes around a baffle 46 and through a chimney in passing from the lower part 31 of casting 40 into the smaller upper part 42. Disposed at or adjacent to one end of the slit or elongated opening is a filamentary cathode 51, across the terminals of which a suitable voltage is impressed when the apparatus is in operation and as will presently be described. The function of the cathode 51 in operation is to emit a stream of electrons into and along the slit 50, the electrons being collimated into a beam under the influence of the magnetic field previously described and serving to bombard the vapor passing through the slit for the purpose of ionizing the vapor.

To the left of the casting 40, that is above it so to speak, is a member 56 which may preferably be made of carbon and which has therein a slot or elongated opening parallel to the slit Stl so that ions of the vapor emitted through the slit 50 can pass through the slit in member 56. The member 56 constitutes an electrode which serves as an accelerating electrode for positive ions emerging through the slit 50, and this electrode is maintained at a relatively high negative potential as will presently be described. It is supported from insulator 59.

The ions accelerated by the electrode 56 pass to the left, that is upwardly through another slit 57 in a member 58 having a throat-like configuration and which also forms an electrode which may preferably be made of carbon. This electrode is also maintained at a relatively high negative potential but lower than the potential of the electrode 56. Positive ions, after passing through the throat of member 58, pass into the liner structure 35, and then travel in arcs of a circle around through the liner to the receiving structure at the opposite side of the liner; the positive ions move in arcs of a circle in this manner under the influence of the magnetic field previously described, and the radii of the arcs depends on the mass-charge properties of the ionized particles.

There are two identical receiving chambers or collectors at the upper end of the liner structure 35, designated by the numerals 6t and 60', one being associated with each of castings 40 and 40. The receiving chamber 60' is in the form of a box having a shape which in cross section is shown in Fig. l. The receiver or collector 60 may be adjusted laterally relative to the face plate by a stem 61' operating through linkages 62', and it may be adjusted in and out relative to the face plate by means of stem 63, the stems passing through suitable sealing devices 64 and 65', respectively, associated with the face plate 11. The collector 60 has electrodes 71' and 72 therein which are so located as to receive certain components of the beam of ions which travels around through the liner 35, electrode 71' being in the form of a pocket. Difierent components of the beam, that is, particles thereof having different mass-charge properties travel in circles of different radii as described above and come to a focus at different radial distances from their point of origin. The electrodes 71 and 72 are supported by insulators 73 and 74' respectively. In operation of the apparatus the magnetic field is so adjusted and the collector 60 is so adjusted relative thereto that the isotopes which it is desired to collect impinge upon the desired receiving element. The desired isotopes may be recovered by removing the deposits which collect in or on the electrodes 71 and 72'.

The face plate 11 is grounded, as shown, as are the castings 40 and 40', and the liner 35, and the electrode 58 are maintained at a relatively high negative potential V1 by conductor 75 which is led into the interior of the tank through a conduit 76 which extends through the face plate 11 through a sealing insulator bushing 77. The electrode 56 is maintained at a higher negative potential V1+V2 by means of a conductor 78 which is also led into the tank through the bushing 77 and the conduit 76. The electrodes 71 and 72 are maintained at the potential V1 by means of conductors 79, 80, and 81. The conductors leading to the electrodes in the collector 60 extend through a conduit and bushing similar to those for the conductors 75 and 788 as shown. A potential V3 is impressed across the terminals of the cathode 51 and a voltage V4 is impressed between the negative terminal of cathode 51 and ground, that is, the casting 40. This last potential sustains the are which is struck within the slit during operation.

Summarizing the operation of the apparatus as so far described, the operation is that the charge material in the bottle 43 is vaporized by the electric heating element 52, and by heating element 52, speaking of casting 40, and the vaporization is automatically controlled by the thermocouples in the wells 53 and 53. The vapor passes upwardly into the upper portion 42 of the casting 40 where it is maintained in vapor state by the portion of heating element 52 associated therewith, and the vapor then passes out or upwardly through the slit 50 where it is ionized by the stream of electrons from the cathode 51, an are being formed or struck in the slit 50 under the influence of voltage V4,. Positive ions are attracted from the region of the slit by electrode 56, which is maintained at a high negative potential, and under the influence of electrode 58 the ions pass into the liner structure 35 and thence travel in arcs of a circle around to the collector 60, the radii of the arcs depending upon the mass-charge properties of the ionized particles. The collector 60 is adjusted as described so that the desired isotopes of the ionized material are received in the collector pockets.

Various of the parts of the mechanism within the tank 10 may be cooled as desired or necessary by a suitable cooling system, and shielding may be provided at appropriate points to protect the mechanism from becoming coated as a result of being contacted by the vapor from the charge bottle, and to provide protection against deterioration which may necessarily result from the process.

The geometry of the slit 50 relative to the other parts is quite critical, and during operation vaporized mate rial may condense and collect on the defining edges of the slit Or the slit may become otherwise fouled up or contaminated. It is necessary and desirable during operation that the slit be kept free from contaminating material insofar as possible. In order to accomplish this I provide a cleaner or scraper mechanism which may be manually operated for cleaning or scraping the defining edges of the slit and the material adjacent to the defining edges. The slit cleaner mechanism is shown diagrammatically in its operative position relative to the other parts in Fig, 1, the cleaner mechanism being operated by a manually operative push rod orplunger 100 having a handle 1111 outside of the face plate 11, the push rod 1110 extending through a Wilson seal 102 into the inte- The mechanism of the slit cleaner enlarged perspective in Fig. 3 and reference is now made more particularly to Fig. 3 of the drawings. The slit cleaner mechanism embodies cleaning or scraping elements which move back and forth in the slits 5t) and 5t) and the cleaning or scraping arrangements are operated by a mechanism which causes them to move substantially in a straight line and which is disposed in a position between the castings 4t) and as may be seen both in Figs. 1 and 3. Referring to Fig. 3, the cleaning or scraping device associated with slit in casting 46) comprises an element or block apparatus is shown in made up of two similar attached pieces which fits in or engages in the slit and is attached to a horizontal cleaner or scraper element or plate 106 which has a slanting surface at its forward edge as shown, and the upper surface of which engages the surface which defines the bottom of the slit 5t Numeral 107 designates a horizontal guide member attached to a member 113 upstanding from plate 106, the ends of which guide member bear against the surface defining the top of the slit or elongated opening 50. The member 113 is attached to a horizontal transverse cross arm 112 as shown. The r cleaning or scraping element associated with the casting 40 is identical with that just described, the cleaning or scraping arrangements associated with the castings 40 and 49' being equally spaced from the midpoint of the cross arm 112. Numeral 117 designates a lever which carries at its upper end transversely thereof a cylinder 114 forming a journal or bearing for a shaft 116 extending therethrough. Attached to cross arm 112 are equally spaced brackets 115 having vertical slots as shown in which the ends of shaft 116 engage. This structure forms a lost motion connection between lever 117 and the cleaning elements which is for a. purpose which will presently be described.

The lower end of lever 117 is pivoted as shown to a link 1118, the opposite end or" which engages on a fixed pivot formed by a bolt 119 extending through bracket 126. The lever 117 extends through a guide slot parallel to the slits 5t) and 50', the guide slot 125 being in a structure 126 which may be built out from the cast ings 411 and 40 or which may be supported directly from the supporting structure 36 of Fig. 1.

At its upper end within guide slot 125 lever 117 has an integral frame-like extension 133 lying in the same plane. Extension 133 has a portion as shown which is substantially parallel to lever Lever 117 is thus guided in slot 125 by guiding surfaces spaced along the length of the guide slot whereby lever 117 is more accurately guided and particularly twisting of lever 117 in guide slot 125 is prevented. Difiiculties occasioned by one cleaning element tending to get ahead of the other and cause binding are thus avoided. Extension 133 is of such configuration that during the straight line movement of the upper end of lever 117 the portion parallel to lever 117 remains in slot 125.

The frame structure 126 has pivotally attached thereto by a bolt 123 a triangle member 129 which is spaced from the side of structure 126 by a bushing through which the bolt 128 extends. The bolt 128 extends through one corner of triangle 129; another corner of the triangle is pivoted to an intermediate point of lever 117; and the opposite corner of triangle 129 is pivotally attached as shown to a link 132, the opposite end of which is pivotally attached to the push rod or operating stem 1011 which extends through the face plate 11 as previously described.

117 but spaced therefrom.

' just described.

' it is convenient that During operation of the apparatus of Fig. 1, the push rod or stem 1150 may be reciprocated periodically at intervals from outside of the tank 10 for cleaning or scraping the slits 50 and 50' to free them of fouling or contaminating material. In the operation of the cleaning mechanism when the handle 101, that: is the stem 100, is pushed upwardly (on Fig. 3), the triangle 129 is rotated in a clockwise direction looking at Fig. 3, that is, link 132 pushes up on the corner of triangle 129. This causes the lever 117 to rotate counterclockwise about the pivot at its lower end, moving the cross arm 112 and its associated cleaning or scraping elements to the left looking at Fig. 3. As lever 117 rotates counterclockwise, its intermediate point which is pivoted to a angle 129 is constrained to move in an arc of a circle, but its lower end moves downwardly, with the link 118 rotating in a clockwise direction looking at Fig. 3. The relationship of the centers, that is, the pivots and the lengths of the lever arms are such that as lever 117 is thus rotated counterclockwise its upper end moves to the left, looking at Fig. 3, in substantially a straight line so that the cleaning or scraping elements have a substantially straight line motion in the slits 50 and 5d, the motion being guided by the guide slit 125 in which the lever 117 moves. When lever 117 reaches a position in which it is perpendicular to the link 118, continued counterclockwise movement of lever 117 will cause link 118 to then move in a counterclockwise rather than a clockwise direction. When the stem 199 has been thus fully pushed in, the cleaning or scraping elements in the slits 5t) and 5'19 will have been moved to the opposite ends of the slits, and in order to move them back, the handle 101 and the operating stem 1% are pulled out. causes the parts to move in opposite directions to that Reciprocating the operating stem 100 and lever 117, occasioned by the change in the angle between lever 117 and cross-arm 112 and by the fact that the straight line motion may be slightly imperfect. Throughout the angular movement of lever 117 the parallel portion of extension 133 remains in slot 125 so that the lever is effectively guided, the guiding member being extensive enough along the length of the slot to prevent one cleaning element from getting ahead of the other and twisting lever 117.

The operating mechanism for producing straight line motion of the cleaning elements can be made very rugged, and with the guiding arrangement described the cleaning elements accurately move together in a straight line .without binding and without encumbering the immediate vicinity of the slits either with anism.

From the foregoing, those skilled in the art will perceive that I have provided an improved device for cleaning the slits of the ion sources and maintaining them in etficient operating condition. The arrangement provides an eflicient means for operating slit cleaners, particularly since a plurality of cleaning elements be actuated by a single operating stem.

The foregoing disclosure is representative of a preferred form of my invention, and it is intended that it be interpreted in an illustrative rather than a limiting sense, it being intended that the scope of the invention be determined in accordance with the claims appended hereto.

I claim:

1. In apparatus of the character described, in combination, means forming an ion source, said source comprising a chamber having an elongated opening therein,

operating or guiding mechmeans comprising an element movable in said opening for removing contaminating material from the defining edges thereof, said last means comprising mechanism for operating said element and movable in a guide slot parallel to said opening, said mechanism comprising a guide member having relatively great extent in the direction of the slot.

2. In apparatus of the character described, in combination, means forming an ion source, said source comprising a chamber having an elongated opening therein, means comprising an element movable in said opening for removing contaminating material from the defining edges thereof, operating means for said element comprising a member spaced from said opening and movable in a guide slot parallel thereto and said member having a guide portion in said slot and of considerable extent along the length of the slot; I

3. In apparatus of the character described, in combination, means forming an ion source, said source comprising a chamber'having an elongated opening therein, means comprising an element movable in said opening for removing contaminating material from the defining edges there of, said last means comprising mechanism for constraining said elementto move in substantially a straight line in said opening,- said mechanism comprising a member movable in a guide slot and having a guide portion of relatively great extent in the direction of the slot.

4. in apparatus of the character described, in combination, means forming a plurality of ion sources, each of said sources comprising a chamber having an elongated opening therein, said elongated openings being parallel, means comprising an element movable in each of said openings for removing contaminating material from the defining edges thereof, means comprising mechanism for operating said elements, said mechanism comprising a member movable in a guide slot parallel to said openings and having-a guide portion of relatively great extent in the dire'ctionof the guide slot.

5. In apparatus of the character described, in combination, means forming a plurality of ion sources, each of said sources comprising a chamber having an elongated opening therein, said elongated openings being parallel, means comprising an element movable in each of said openings for removing comtaminating material from the defining edges thereof, means comprising mechanism for operating said elements, said mechanism being operatively connected to all of the elements and arranged to constrain said elements to move in substantially a straight line in said openings, said mechanism comprising a member movable in a guide slot and having a guide portion of relatively great extent in the direction of the slot.

6. In apparatus of the character described, in combination, means forming a plurality of ion sources, each of said sources comprising a chamber having an elongated opening therein, said elongated openings being parallel, means comprising, an element movable in each of said openings for removing contaminating material from the defining edges thereof, means comprising mechanism for operating said elements, said mechanism being operatively connected to all of the elements and arranged to constrain said elements to move in substantially a straight line in said openings, saidmechanism comprising a lever on a fixed pivot, a second lever pivoted to the first lever, and a third lever on a fixed pivot and pivoted to said second lever so that when the third is rotated, the first and second levers rotate on their pivots with a portion of the second lever moving substantially in a straight line, and means whereby said second lever is guided, said last means hav ing guiding surfaces spaced over a relatively great area so as to cause said second lever to move in a single plane without twisting or wobbling out of said plane.

7. In apparatus of the character described, in combination, means forming an ion source, said source comprising a chamber having an elongated opening therein, means comprising an element movable in said opening for removing contaminating material from the defining edges thereof, said last means comprising mechanism for constraining said element to move in substantially a straight line in said opening, said mechanism including a lever and a pivot which said lever is rotatable about as center, means for rotating said lever about said center, and for also moving said center in a manner whereby a portion of said lever moves in substantially a straight line, and means whereby said lever is guided, said last means having guiding surfaces spaced over a relatively great area so as to cause said lever to move in a single plane without twisting or wobbling out of said plane.

8. in apparatus of the character described, in combination, means forming a plurality of ion sources, each of said sources comprising a chamber having an elongated opening therein, said elongated openings being parallel, means comprising an element movable in each of said openings for removing contaminating material from the defining edges thereof, means comprising mechanism for operating said elements, said mechanism being operatively connected to all of the elements and arranged to constrain said elements to move in substantially a straight line in said openings, said mechanism comprising a lever mounted on a pivot so as to be rotatable about said pivot as center, means for rotating said lever about said center and for moving said center in a manner whereby a portion of said lever moves in substantially a straight line, and means whereby said lever is guided, said last means having guiding surfaces spaced over a relatively great area so as to cause said lever to move in a single plane without twisting or wobbling out of said plane.

9. In apparatus of the character described, in combination, means forming a plurality of ion sources, each of said sources comprising a chamber having an elongated opening therein, said elongated openings being parallel, means comprising an element movable in each of said openings for removing contaminating material from the defining edges thereof, mechanism for operating said elements, said mechanism being operatively connected to all of the elements and arranged to constrain said elements to move in substantially a straight line in said openings, said mechanisms being positioned at least in part between two of said ion sources and comprising a lever and a cross arm extending transversely of the lever and guide means for the lever, said guide means having guiding surfaces spaced over a relatively great area so as to cause said lever to move in a plane parallel to said openings without twisting out of said plane.

10. In apparatus of the character described, in combination, means forming a plurality of ion sources, each of said sources comprising a chamber having an elongated opening therein, said elongated openings being parallel, means comprising an element movable in each of said openings for removing contaminating material from the defining edges thereof, mechanism for operating said elements, said mechanism being operatively connected to all of the elements and comprising a lever rotatable on a pivot and lying in a plane parallel to said openings and between two of said sources, means for rotating said lever on its pivot and for also moving said pivot in a manner to cause a portion of the lever to move in substantially a straight line, and means whereby said lever is guided, said last means having guiding surfaces spaced over a relatively great area so as to cause said lever to move in a single plane without twisting or wobbling out of said plane.

11. In apparatus of the character described, in combination, means forming an ion source, said source comprising a chamber having an elongated opening therein, means comprising an element movable in said opening for removing contaminating material from the defining edges thereof, said last means comprising mechanism for constraining said element to move in substantially a straight line in said opening, said mechanism comprising means forming two members mounted on fixed pivots so as to be 9 rotatable thereabout, means forming a lever pivoted to both of said members, and means for rotating one of the members so that the lever rotates about its pivot on the other member and the other member also rotates, the lever having a portion which moves in substantially a straight line, and means for guiding said lever, said last said sources comprislng a chamber having an elongated opening therem, said elongated openings being parallel,

means comprising an element movable in each of said openings for removing contaminating material from the defining edges thereof, means comprising mechanism for said second lever, said guide means including a guide slot and said second lever having guide surfaces spaced over a considerable area so as to prevent twisting of the lever in the slot.

15. In apparatus of the character described, in combielement to move in substantially a straight line in said openin said mechanism including a lever and a pivot which said lever is rotatable about as center,

No references cited. 

1. IN APPARATUS OF THE CHARACTER DESCRIBED, IN COMBINATION, MEANS FORMING AN ION SOURCE, SAID SOURCE COMPRISING A CHAMBER HAVING AN ELONGATED OPENING THEREIN, MEANS COMPRISING AN ELEMENT MOVABLE IN SAID OPENING FOR REMOVING CONTAMINATING MATERIAL FROM THE DEFINING EDGES THEREOF, SAID LAST MEANS COMPRISING MECHANISM FOR OPERATING SAID ELEMENT AND MOVABLE IN A GUIDE SLOT PARALLEL TO SAID OPENING, SAID MECHANISM COMPRISING A GUIDE MEMBER HAVING RELATIVELY GREAT EXTENT IN THE DIRECTION OF THE SLOT. 